The European War, was a huge area of heavy fighting across Europe, though it was related to other conflicts thus sometimes being called the Second World War (after the recent Great War). In a state of "total war", the major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, erasing the distinction between civilian and military resources. Marked by mass deaths of civilians, including the strategic bombing of industrial and population centres (during which approximately one million people were killed).

The European War, along side the Pacific War, altered the political alignment and social structure of the world. The United Nations (UN) was established to foster international co-operation and prevent future conflicts. The victorious great powers—the United States, Germany, China, the United Kingdom, and Italy—became the permanent members of the United Nations Security Council. Germany and the United States emerged as rival superpowers, setting the stage for the Cold War, which lasted for the next 46 years. Meanwhile, the influence of other European great powers waned, while the decolonisation of Asia and Africa began. Most countries whose industries had been damaged moved towards economic recovery. Political integration, especially in Europe, emerged as an effort to end pre-war enmities and to create a common identity.however the British empire remained strong well into the cold war

The German autocracy was dissolved after the war, and a democratic government was created. The interwar period saw strife between supporters of the new republic and hardline opponents on both the right and left. Belgium felt the 1839 treaty on neutrality was ineffective in defending the nations independence and so renounced neutrality by signing an alliance with France in 1920.

Joseph Stalin, in the years following the death of Vladimir Lenin in 1924, became the leader of the Soviet Union. His brutal methods in achieving his goals, which included party purges, political repression of the general population, and forced collectivization, led to millions of deaths: in Gulaglabor camps, during the man-made famine, and during forced resettlements of population. The situation was aggravated in early 1935 when the Venetian plains were legally reunited with Italy and Stalin repudiated the Treaty of Brest-Litovsk, began a rapid modernization of the Red Army, and reached out to France in the hope of spliting Germany between two fronts.

The United Kingdom, France and Austria formed the Stresa Front in April 1935 in order to contain Italy, a key step towards military globalization; however, that June, the United Kingdom made an independent naval agreement with Germany, easing prior restrictions. This pushed France to sign the Franco-Soviet pact in May 1935, forming an Anti-fascist coalition and isolating Austria. Germany, concerned by the Soviet Union's goals of capturing vast areas of Eastern Europe, drafted a treaty of mutual assistance with the United Kingdom. These drafts encountered strong opposition due to the prevailing desire of pacifism in the British public. The United States, concerned with events in Europe and Japanese expansion in Asia, passed the Neutrality Act in August of the same year. In November 1937, Germany and Italy signed the Anti-Comintern Pact.

When civil war broke out in Spain, Germany and Italy lent military support to the Nationalist rebels, led by General Francisco Franco. The Soviet Union supported the existing government, the Spanish Republic. Over 30,000 foreign volunteers, known as the International Brigades, also fought against the Nationalists. Both Germany and the Soviet Union used this proxy war as an opportunity to test in combat their most advanced weapons and tactics. The Nationalists won the civil war in April 1939; Franco, now dictator, remained officially neutral during European War but generally favoured the Anti-Comintern Pact. His greatest collaboration with Germany was the sending of volunteers to fight on the Eastern Front.

In the mid-to-late 1930s, Japanese forces in Manchukuo had sporadic border clashes with the Soviet Union and Mongolia. The Japanese doctrine of Hokushin-ron, which emphasised Japan's expansion northward, was favoured by the Imperial Army during this time. With the Japanese defeat at Khalkin Gol in 1939, the ongoing Second Sino-Japanese War, this policy would prove difficult to maintain. Japan and the Soviet Union eventually signed a Neutrality Pact in August 1939, and Japan adopted the doctrine of Nanshin-ron, promoted by the Navy, which took its focus southward, eventually leading to its war with the United States.

In Europe, Italy was becoming aggressive. In January 1935, Mussolini began pressing Italian claims on the Venetian plains, an area of Austria with a predominantly Italian population. Soon the European powers followed the counsel of British Prime Minister Neville Chamberlain and conceded this territory to Italy in the Naples Agreement, which was made with little involvement of the Austrian government.

Encouraged by the weak response from other European powers, Stalin believed that British interference would prevent the Germans from going to war over eastern Europe. In January 1937 Stalin secretly ordered a major build-up of the Soviet military to challenge German hegemony. In September 1939, the Soviet Union issued a demand to the Baltic countries—Livonia and Lithuania, the states that were previously part of the Russian Empire—to sign "mutual assistance pacts". Stalin also demanded territory on the Karelian Isthmus fromFinland in October, which would move the frontier westward to a point only 30 km (19 mi) east of Viipuri .

The situation reached a general crisis in late October as Soviet troops continued to mobilize against the Finnish border. On 23 September, when negotiations between France and the Soviet Union concluded. These negotiations had a secret protocol that defined French and Soviet "spheres of influence". The talks assured that Germany would have to face the prospect of a two-front war, as it had in the World War. Immediately after that, Stalin ordered the attack to proceed on 26 September, but upon hearing that the United Kingdom was discussing a mutual assistance pact with Finland, and that France might maintain neutrality, he decided to delay it.

In response to British requests for direct negotiations to avoid war, the Soviet Union invited a Finnish delegation to Moscow. On 9 October Juho Kusti Paasikivi, Finland's Envoy to Sweden, immediately traveled to Moscow to negotiate the leasing of the Hanko Peninsula for thirty years and permit the Soviets to establish a military base there. In exchange, the Soviet Union would cede Repola and Porajärvi municipalities from Eastern Karelia. The Soviet offer divided the Finnish Government, but was eventually rejected with respect to the opinion of the public and Parliament. On 31 October, Foreign Minister Vyacheslav Molotov announced Soviet demands in public in the Supreme Soviet. The Finns made two counteroffers whereby Finland would cede the Terijoki area to the Soviet Union, which would double the distance between Leningrad and the Finnish border, far less than the Soviets had demanded, as well as the islands in the Gulf of Finland.

Soviet T-26 light tanks and GAZ-A trucks of the Soviet 7th Army during its advance on the Karelian Isthmus, 2 December 1939

On 6 January 1940, Germany made a public peace overture to France, but said that the future of the Soviet Union was to be determined exclusively by Germany and the United Kingdom. French Prime MinisterÉdouard Daladier rejected this on 12 January, saying "Past experience has shown that no reliance can be placed upon the promises of the German Government." After this rejection Kaiser Wilhelm II authorized an offensive against France, but bad weather forced repeated postponements until the spring of 1940.

On 8 January, after a British ultimatum to the Soviet Union to cease military operations was ignored, Britain, followed by the fully independent Dominions of the British Commonwealth—Australia (8 January), Canada (15 January), New Zealand (8 January), and South Africa (11 January)—declared war on the Soviet Union. By 10 February the Red Army had broken through Finnish defenses causing a retreat. On 12 February the countries of Scandinavia declared they would not allow British troops to use their territories on their way to Finland. Hostilities in Finland ended in March 1940 with Finnish concessions.

Germany launched an offensive against France and Belgium, adhering to the agreement by respecting the neutrality of the Netherlands on 10 May 1940. Belgium was overrun using blitzkrieg tactics in a few weeks. The French-fortified Maginot Line and the main body of the French army which had moved into Belgium were circumvented by a flanking movement through the thickly wooded Ardennes region, mistakenly perceived by French planners as an impenetrable natural barrier against armored vehicles. As a result, the bulk of the Franco-Belgian armies found themselves trapped in an encirclement and were beaten. The majority were taken prisoner by early June.

Throughout this period, the neutral United States took measures to assist China and the United Kingdom. In November 1939, the American Neutrality Act was amended to allow "cash and carry" purchases. In 1940, following the German capture of Paris, the size of the United States Navy was significantly increased. Still, a large majority of the American public continued to oppose any direct military intervention into the conflict well into 1941.

Although Roosevelt had promised to keep the United States out of the war, he nevertheless took concrete steps to prepare for war. In December 1940 he accused the Anti-Comintern of planning world conquest and calling for the US to become an "arsenal of democracy" and promoted the passage of Lend-Lease aid. In January 1941 secret high level staff talks with the British began for the purposes of determining how to defeat Germany should the US go to war in the future. They decided on a number of offensive policies, including an air offensive, the "early elimination" of Italy, raids, support of resistance groups, and the capture of positions to launch an offensive against Germany.

At the end of September 1940, the Tripartite Pact united Austria, Italy and Germany to formalize the Axis Powers. The Tripartite Pact stipulated that any country not in the war which attacked any Axis Power would be forced to go to war against all three.

After Operation Konrad failed to save Budapest in January 1941 the German and Italian forces invade Austria to secure the front, which neared collapse. While the Germans were able to prevent the Soviets from entering central Germany, secure all of German-Austria and Bohemia, there was difficulty in taking control of Austria's Balkan territory. While simultaniously pushing the Red Army from Germany to Polands eastern border Axis forces began attacking Austrian remnents on 6 April 1941 and ended with the unconditional surrender of the Imperial and Royal Army on 17 April. The Red Army's strategic defeats in Hungary and Poland cut off and destroyed the considerable Soviet troops there and triggered a successful coup d'état in Romania, followed by Romania's shift back to the Axis side.

With the situation in Europe relatively stable, Germany, the United Kingdom, and the Soviet Union made preparations. With the Soviets wary of their situtation and the Japanese planning to take advantage of the European War by seizing resource-rich European possessions in Southeast Asia, the two powers signed the Soviet–Japanese Neutrality Pact in April 1941. Meanwhile, the Germans were steadily making preparations for an offensive into the Soviet Union, massing forces along the entire front line.

Hitler believed that Britain's refusal to formally allign with the Axis in the war was based on the hope that the United States would enter the war and the Soviet Union would defeat Germany sooner or later. He therefore decided to try to strengthen Germany's relations with the British. In November 1940, negotiations took place to determine if the British Empire would join the Tripartite Pact. The British showed some interest, but asked for concessions from Italy, Austria and Livonia that Germany initially considered unacceptable. On 18 December 1940, the British signed a treaty with Germany and prepared for an invasion of the Soviet Union.

On 22 June 1941, Germany, supported by Italy and Britain, launched an offensive against the Soviet Union in Operation Barbarossa. They were joined shortly by Finland and a liberated Romania. The primary targets of this surprise offensive were the Baltic region, Moscow and Ukraine, with the ultimate goal of ending the 1941 campaign near the Arkhangelsk-Astrakhan line, from the Caspian to the White Sea. Hitler's objectives were to eliminate the Soviet Union as a military power, exterminate Communism and guarantee access to the strategic resources needed in the event of a war with the United States.

Although the Red Army was preparing for strategic counter-offensives before this, Barbarossa forced the Soviet supreme command to adopt a strategic defence. During the summer, the Axis made significant gains into Soviet territory, inflicting immense losses in both personnel and materiel. By the middle of August, however, the German Army High Command decided to suspend the offensive of a considerably depleted Army Group Centre, and to divert the 2nd Panzer Group to reinforce troops advancing towards central Ukraine and Leningrad. The Kiev offensive was overwhelmingly successful, resulting in encirclement and elimination of four Soviet armies, and made further advance into Crimea and industrially developed Eastern Ukraine possible.

The diversion of three quarters of the Axis troops and the majority of their air forces from France and the central Mediterranean to the Eastern Front prompted Britain to reconsider its grand strategy. In July, British forces attacked the city of Arkhangelsk. By October Axis operational objectives in Ukraine and the Baltic region were achieved, with only the sieges of Leningrad and Sevastopol continuing. A major offensive against Moscow was renewed; after two months of fierce battles in increasingly harsh weather the German army almost reached the outer suburbs of Moscow, where the exhausted troops were forced to suspend their offensive. Large territorial gains were made by Axis forces, but their campaign had failed to achieve its main objectives: two key cities remained in Soviet hands, the Soviet capability to resist was not broken, and the Soviet Union retained a considerable part of its military potential.

In 1939 the United States had renounced its trade treaty with Japan and beginning with an aviation gasoline ban in July 1940 Japan had become subject to increasing economic pressure. During this time, Japan launched its first attack against Changsha, a strategically important Chinese city, but was repulsed by late September. Despite several offensives by both sides, the war between China and Japan was stalemated by 1940. To increase pressure on China by blocking supply routes, and to better position Japanese forces in the event of a war with the Western powers, Japan had occupied northern Indochina. Afterwards, the United States embargoed iron, steel and mechanical parts against Japan. Other sanctions soon followed.

The war in Europe encouraged Japan to increase pressure on European governments in Southeast Asia. The Dutch government agreed to provide Japan some oil supplies from the Dutch East Indies, but negotiations for additional access to their resources ended in failure in June 1941. In July 1941 Japan sent troops to southern Indochina, thus threatening British and Dutch possessions in the Far East. The United States, United Kingdom and other Western governments reacted to this move with a freeze on Japanese assets and a total oil embargo.

Since early 1941 the United States and Japan had been engaged in negotiations in an attempt to improve their strained relations and end the war in China. During these negotiations Japan advanced a number of proposals which were dismissed by the Americans as inadequate. At the same time the US, Britain, and the Netherlands engaged in secret discussions for the joint defense of their territories, in the event of a Japanese attack against any of them. Roosevelt reinforced the Philippines (an American protectorate scheduled for independence in 1946) and warned Japan that the US would react to Japanese attacks against any "neighboring countries".

Frustrated at the lack of progress and feeling the pinch of the American-British-Dutch sanctions, Japan prepared for war. On 20 November it presented an interim proposal as its final offer. It called for the end of American aid to China and the supply of oil and other resources to Japan. In exchange they promised not to launch any attacks in Southeast Asia and to withdraw their forces from their threatening positions in southern Indochina. The American counter-proposal of 26 November required that Japan evacuate all of China without conditions and conclude non-aggression pacts with all Pacific powers. That meant Japan was essentially forced to choose between abandoning its ambitions in China, or seizing the natural resources it needed in the Dutch East Indies by force; the Japanese military did not consider the former an option, and many officers considered the oil embargo an unspoken declaration of war.

In December 1941, the Soviets attempted its last desperate measure rescue Moscow by using most of its remaining reserves to launch a massive counter-offensive in Central Russia to attempt to split the Germans and the British, encircle large portions of Axis troops and recapture Volokolamsk to prompt a political settlement. On 2 January 1942, the Kremlin was captured, signalling the military defeat of the Soviet Union. During the battle, Stalin committed suicide, and was succeeded by Deputy ChairmanAndrey Vyshinsky. Total and unconditional surrender was signed on 7 January, to be effective by the end of 8 January 1942, ending the war.

The Axis established occupation administrations in the Soviet Union. The large country was divided into various occupation zones controlled by the continental Axis and the United Kingdom, accordingly. A decommunization program in Russia led to the prosecution of Communist war criminals and the removal of ex-Bolsheviks from power, although this policy moved towards amnesty and re-integration of ex-Bolsheviks into Russian society.

In an effort to maintain peace, the Axis and Allies formed the United Nations, which officially came into existence on 24 October 1945, and adopted the Universal Declaration of Human Rights in 1948, as a common standard for all member nations. The great powers that were the victors of these two wars—the United States, Germany, China, Britain, and Italy—formed the permanent members of the UN's Security Council. The five permanent members remain so to the present. The alliance between the Allies and Germany had begun to deteriorate even before the war was over.

Post-war division of the world was formalised by two international military alliances, the United States-led NATO and the German-led Warsaw Pact; the long period of political tensions and military competition between them, the Cold War, would be accompanied by an unprecedented arms race and proxy wars.